1. Field of the Invention
This invention relates to electrodeposition of chromium on basis metals, and, more particularly, it is concerned with an activation process by which an adherent chromium electrodeposit is formed on ferrous metal substrates using a high energy efficient chromium plating bath.
2. Description of the Prior Art
The use of high efficiency chromium plating processes has been hampered by the inability to obtain adequate coating adhesion to certain basis metals.
Chromium plating baths containing halides have exhibited adhesion problems. Such baths are of the types disclosed in Mitsui, J7B-33941 (September, 1978); Dillenberg U.S. Pat. No. 4,093,522; Perakh et al., U.S. Pat. No. 4,234,396; and Chessin, U.S. Pat. Nos. 4,450,050 and 4,472,249.
The problem has been attributed to the presence of a halide which may interfere at the initiation of deposition.
In order to obtain an adequate bond as measured by ASTM B 571-79, with typical chromium plating solutions, such as those using a solution of chromic acid and catalysts such as sulfate or sulfate in combination with various fluorides, the usual technique is to reverse or anodically etch a ferrous workpiece in the plating solution or in a separate chromium acid containing solution at a predetermined current density for a predetermined time.
A table which lists the time lengths for such an etching process is found in "Metal Finishing" 80 (5) 65-8 (1982) by C. H. Peger. The use of certain sulfuric acid and hydrofluoric acid etches for specific stainless steels is also suggested in the publication.
Anodic chromic acid treatments for 400 stainless steel alloys and for low and high carbon steels is disclosed in "48th Metal Finishing Guidebook-Directory" 78, 188-202 (1980) by A. Logozzo. Also recommended are cathodic treatments in sulfuric-fluoride solutions for 300 stainless, for nickel alloys and for cast iron.
A further alternative which is found in the prior art is the use of a Woods nickel strike for certain special nickel and cobalt-based alloys. (Boeing Aircraft) BAC5709-5.2j(1).
The use of a ferric chloride-hydrochloric acid solution as a clearing agent for the smut produced by anodic sulfuric etches is disclosed at page 137 of "Hard Chromium Plating" Robert Draper Ltd., Teddington, 1964 by J. D. Greenwood.
Brune and McEnally in "Plating"42, 1127-32 (1955) describe the use of a magnesium sulfate sulfuric acid anodic etch solution for preparing ferrous parts for plating. Similarly, ASTM Specification B-242-49T suggests the application of an anodic etch using a sulfuric acid solution containing sodium sulfate. ASTM B177-68 described the use of sulfuric acid or chromic acid as activators for chromium electroplating on steel for engineering use.
Chessin in U.S. Pat. No. 4,450,050 described an activation pretreatment for bonding high efficiency chromium electrodeposits on a metal substrate which is characterized by the step of first plating the substrate metal with iron or an iron alloy from an iron salt containing bath.
Hermann, in U.S. Pat. No. 4,416,758, activates metal substrates in an aqueous alkaline cyanide containing solution using current which is periodically reversed, followed by rinsing and chromium plating.
It has been found that when these procedures are employed with the halide containing chromium plating baths, most of the ferrous metal substrates are not adequately plated in that the chromium deposits from these baths have inadequate adhesion.
It can be speculated that the reducing conditions at the cathode at the initiation of deposition cause the halide ion to be reduced to a form which interferes with the molecular bonding of the chromium to the substrate. In any event the use of high efficiency chromium plating is limited by the problem of inadequate adhesion.
Accordingly, it is an object of this invention to provide an improved process for forming adherent chromium electrodeposits, particularly from high energy efficient baths, on ferrous metal substrates.
Another object herein is to provide an activation solution for pretreating a ferrous metal substrate before electrodepositing chromium thereon, which solution does not require the use of strong acids such as sulfuric acid, which would effect the ferrous metal substrates.
Still another object is to provide an activation solution for electrolytically, preferably anodically, etching a ferrous metal substrate in preparation for electrodepositing an adherent chromium metal deposit thereon from a high energy efficient bath, where the degree of adherence of the chromium deposit is relatively independent of the transit time between activation and rinsing.
Yet another object herein is to provide adherent and smooth chromium deposits on a ferrous metal substrate from a high energy efficient chromium electroplating bath.